The up-coming strict emission legislation demands new and improved catalysts for diesel vehicle deNOx. The demand for low-temperature activity is especially challenging. H2-assisted NH3-SCR over Ag/Al2O3 has shown a very promising low-temperature activity and a combination of Ag/Al2O3 and Fe-BEA can give a high NOx conversion in a broad temperature window without the need to dose H2 at higher temperatures. The aim of this study has been to investigate the combined Ag/Al2O3 and Fe-BEA catalyst system both at laboratory-scale and in full-scale enginebench testing. The catalysts were combined both in a sequential dual-bed layout and a dual-layer layout where the catalysts were coated on top of each other. The Ag/Al2O3 catalyst was also investigated with the aim of improving the sulphur tolerance and low-temperature activity by testing different alumina-supports. A large focus of this study has been the preparation of monolithic catalyst bricks for the catalyst testing. A high SBET and higher Ag loading gave a high sulphur tolerance and activity. It was believed that the high SBET is needed to give a higher NH3 adsorption capacity, necessary for the SCR reaction. A higher Ag loading gives more Ag sites and probably a favourable Ag dispersion. Testing with sulphur gave an increased activity of the catalysts. Testing of monolithic catalysts showed a similar activity enhancement after a few standard test cycles. A change in the dispersion or state of Ag can be possible reasons for the activation seen and the activation was believed to be related to Ag and not the alumina. Small-scale laboratory testing showed that it was preferred to have Ag/Al2O3 either upstream or as the outer layer of Fe-BEA. This was attributed to complete NH3 oxidation over Fe-BEA giving a deficit of NH3 over the Ag/Al2O3 if it was placed downstream or as the inner layer. Full-scale engine testing, on the other hand, showed the opposite for a dual-brick layout. High NO2 concentrations are believed to give fast-SCR over the Fe-BEA when it was placed upstream of the Ag/Al2O3. The activity of the combined catalyst layouts were higher than the activity for individual catalysts when less or no H2 was co-fed in the small-scale case showing that there were synergistic effects by combining them. The dual-layer layout showed the best performance which is believed to be attributed the short diffusion distance between the layers allowing diffusion of reaction intermediates between them. Ag/Al2O3 only and the combined Ag/Al2O3 – Fe-BEA systems were active during the transient NEDC. The NOx conversions were not very high which is related to the very low temperature of the NEDC and the lower than expected activity of the Ag/Al2O3 catalyst seen in stationary testing. The most interesting result was that the catalyst systems showed NOx conversion already from the start of the cycle, before any NH3 or H2 was dosed. NOx storage over the Ag/Al2O3 was believed to be the most likely explanation for this. The NOx conversion could be enhanced by dosing of NH3 and H2 at temperatures lower than 150°C that was used as standard starting temperature of dosing. However, dosing too early inhibited the NOx conversion.
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Department of Physics, Technical University of Denmark, 2013